Electrophotographic compositions and imaging processes are well known. In these processes an electrophotographic element having a layer containing a photoconductor is electrostatically charged and then imagewise exposed to form a latent electrostatic image. The latent electrostatic image is subsequently developed with a toner composition. Electrophotographic elements and processes are disclosed, for example, in U.S. Pat. Nos. 3,141,770 to Davis et al., 3,554,745 to Van Allen, 3,577,235 to Contois, 3,615,414 to Light et al., 4,442,193 to Chen et al., 4,421,837 to Hiroshi et al., and 4,468,444 to Contois. Unfortunately, with any electrophotographic element, it is always necessary to charge electrostatically and imagewise expose the charged element for each copy being made.
Multiple copies have been made from a single exposure using photoelectrographic elements in which a persistent differential conductivity pattern is created between exposed and unexposed portions. This allows for subsequent use of the element in printing multiple copies from a single exposure with only multiple charging, developing, transferring, and cleaning steps. This is different from electrophotographic imaging techniques where the electrophotographic element must generally be charged electrostatically followed by imagewise exposure for each copy produced.
Photoelectrographic masters are ideal for use in xeroprinting or multiple color proofing, because multiple high-quality prints can be produced rapidly in view of the need for only a single exposure. This is especially useful in making color images.
One type of master, disclosed in U.S. Pat. No. 4,818,660 to Blauchet-Fincher et al. and U.S. Pat. No. 4,859,551 to Kempf, is prepared by coating a photohardenable layer on an electrically conductive substrate and exposing the layer imagewise to light. Exposed portions of the photohardenable layer harden and become nonconductive, while the unexposed parts of the layer remain unhardened and conductive. When the master is electrostatically charged and developed by applying a toner of opposite charge, the toner adheres to exposed areas. Such films, however, are difficult to handle due to the tackiness of unhardened polymer.
Photoelectrographic master elements generally have a conductive layer in electrical contact with a film layer. When exposed to ultraviolet radiation, photochemically-generated charges form in the film, making the film conductive, while unexposed areas of the film remain insulating. When the element is charged, charges at the surface of the element and at the interface between the film and the conductive layers are neutralized where exposure has occurred. Unexposed areas, however, are charged and then developed with toner. The toned image is transferred to a receptor sheet.
In U.S. Pat. No. 4,661,429 to Molaire et al., the film layer includes an aromatic onium salt or a 6-substituted-2,4-bis (trichloromethyl)-5-triazine acid photogenerator, an insulating binder, and, optionally, a sensitizer. U.S. Pat. Nos. 3,879,197 and 3,982,935 to Bartlett et al. describe a photoelectrographic element with a layer containing a binder and organic halogen compounds, capable of forming hydrohalide acids upon illumination with light, in combination with certain organic compounds. U.S. Pat. No. 3,998,636 to Van den Houte, et al. describes an electrostatic printing master in which the recording layer comprises a mixture of an organic polyhalogen compound, a vinylcarbazole copolymer, and an anilide sensitizer which increases the conductivity of the layer on exposure. Research Disclosure 12846 (December 1974) to Peter M. Stacy et al. utilizes a photoconductive composition containing triarylamine type photoconductors, polyester binders, and sensitizers. The photoelectrographic element of U.S. Pat. No. 3,451,811 to Brynko utilizes the photochromic properties (i.e., photo-isomerization) of spirobenzopyrans. U.S. Pat. No. 3,748,128 to McNally describes a layer containing heterocyclic nuclei which, when exposed, form a latent image with changed triboelectric properties suitable for development with a toner. Such films are, however sensitive to humidity, which causes unexposed areas to become more conductive as humidity decreases.